1. Field of the Invention
The present invention relates to propping agents, and more particularly to propping agents that, are labeled to enable detection of the presence of the propping agent.
2. Description of the Related Art
One of the problems encountered in attempting to maximize recovery of hydrocarbons such as crude oil and natural gas from underground formations is the entrapment of hydrocarbons within low permeability formations. In fact, wells often contain large amounts of the hydrocarbon entrapped in such low permeability rock formations. The entrapped hydrocarbon, of course, does not flow readily to the well bore.
Thus, subterranean formations that contain entrapped hydrocarbons are often “fractured” to enhance the recovery of the entrapped hydrocarbon from the formations. Fracturing typically involves the injection of viscosified aqueous or hydrocarbon fluids into the well bore at a rate and pressure in excess of the formation stresses, thereby causing rock fatigue and opening or inducing new fractures in the formation. Fractures are natural or induced fissures or channels in the formation matrix. The injected fluids usually contain a proppant material, commonly referred to as a “propping agent” or simply a “proppant.” Proppants are particulate solids such as sand or ceramic particles, which may or may not be coated with another material such as resin. After the exerted injection pressure has been relieved, the fractures, which would otherwise tend to close, are propped open by propping agent left behind in the fracture. More conductive channels are thus provided to allow the oil or gas to flow to the well bore after the injection pressure is relieved.
Frequently, however, a substantial portion of the proppant does not remain in the fractures, but flows back to the well bore. Such proppant flowback not only results in inefficiency due to the failure of the proppant that has flowed back to serve its purpose of propping open the fractures, but also can cause serious wear in the production equipment. In wells that contain more than one zone to which proppant has been delivered it can be very difficult to determine which of the zones may be the source of the proppant flowback problem. Therefore, the proppant flowback problem is particularly troublesome in such wells.
Some techniques have been developed which provide a means to identify the zone or zones that are the source of the proppant flowback. Generally, such techniques involve tagging the proppants with a tracer or marker that can be detected by some standard method. According to such techniques, the proppant delivered to each zone is tagged with a tracer distinct from the tracers associated with the other zones. By detecting which tracer is present in the proppant that has flowed back from the formation, it can then be determined the zone from which the proppant flowed.
However, none of the techniques so far developed are entirely satisfactory. For example, radioactive tracers have been used, but radioactive materials can have a short shelf-life and may be difficult to handle and can be hazardous to the environment. U.S. Pat. No. 6,691,780 discloses a technique for tagging proppants with non-radioactive materials, but that technique employs a tag within a resin coating over the proppant. Thus, the technique is limited to resin-coated proppants and is susceptible to loss of the tags if the coating is lost by friction, heat or other means.
As a result, superior tagged proppants, and methods of producing them, that avoid the aforementioned problems are still needed. In particular, it is desired that the tagged proppant be non-radioactive and be tagged in a way that is not susceptible to loss of the tracer by friction and the like. Moreover, because the proppants must be suspended in the carrier fluid and must withstand substantial forces to prop open fractures, and because the purpose of the proppants is to increase flow-through or “conductivity” of fluids, the tagged proppant should maintain the strength and density of the untagged proppant, and should provide at least a similar conductivity (that is, fluid flow-through) as does the untagged proppant.